The present invention relates to a skew prevention structure of an electrophotographic printer for preventing the skew of a recording medium due to the winding of the recording medium around a photoconductive drum.
Conventionally, there has been known an electrophotographic image forming apparatus such as a copy machine and printer employing a so-called electrophotographic image forming process. In the electrophotographic image forming apparatus, the uniformly charged photoconductive material provided on the surface of a photoconductive drum is exposed to light, carrying an image data, to form a latent image. The latent image is developed by adhering toner thereto (a toner image is formed), and the toner image is transferred onto a recording medium and fixed.
Some of the electrophotographic printers print images onto a so-called fanfold recording sheet which is a continuous recording sheet provided with perforated tear lines, which are defined at the portions to be folded. Hereinafter, this continuous form recording sheet is simply abbreviated as a continuous sheet. The continuous sheet can be easily cut off at the perforated tear lines.
Incidentally, in the electrophotographic image forming apparatus, a so-called heat roll fixing process is generally employed.
In the heat roll fixing process, a pair of fixing rollers comprising a heat roller heated to a high temperature and a press roller are arranged such that the rotational axis of the press roller is in parallel to that of the heat roller, and the recording sheet carrying an unfixed toner image thereon, is nipped between the pair of fixing rollers. The unfixed toner image on the recording sheet is fused by being heated with a heated roller (heat roller), and fixed onto the recording sheet at a fixing station. The heat roll fixing process is advantageous in that excellent energy efficiency is realized and a fixing speed can be increased.
The fixing station also functions as a feed means to feed the recording sheet nipped between the pair of fixing rollers. Usually, the heat roller is rotatably driven, and the press roller is driven to be rotated in accordance with the rotation of the heat roller.
FIG. 2 shows an example of a laser beam printer for printing images onto a continuous sheet employing the electrophotographic image forming process. The laser beam printer comprises a toner cleaner 2, discharging station 3, charging station 4, scanning optical system 5 for emitting a scanning laser beam onto a photoconductive drum 1, a developing station 6, and a transferring station 7 around the photoconductive drum 1 along the rotational direction thereof. Further, a fixing station 8 is disposed at the position in which the continuous sheet 9 is fed. A tractor 10 is disposed between the photoconductive drum 1 and the fixing station 8. The tractor 10 is driven by the continuous sheet 9 as the continuous sheet 9 is fed. The tractor 10 applies a predetermined amount of tension to the continuous sheet 9 as it is fed from the photoconductive drum 1 to the fixing station 8.
Incidentally, in the electrophotographic image forming apparatus as described above, when a printing ratio is lowered (e.g., 5% or less), the continuous sheet is attracted around the circumferential surface of the photoconductive drum in a relatively wide area and thus a problem arises in that so-called skew occurs (the continuous sheet proceeds obliquely or windingly), wherein the printing ratio is the proportion of the area to which toner is applied to the printable area on the continuous sheet 9.
At the transferring station in the electrophotographic image forming apparatus, a recording sheet is charged to a reverse polarity with respect to that of the toner attracted on the photoconductive drum by a charger, such as a corona charger or the like, so that the toner on the surface of the photoconductive drum is electrically attracted and transferred to the recording sheet.
In the case of discharged area development as in the printer, the surface of the photoconductive drum is charged at the same polarity as that of the toner. When a printing ratio is low, and accordingly, the amount of the toner attracted onto the circumferential surface of the photoconductive drum is small, the recording sheet easily attracted to the circumferential surface thereof. However, the amount of the recording sheet attracted to the photoconductive drum (winding length, or the length of the area of the recording sheet winding around or contacting the circumferential surface of the photoconductive drum) depends upon the amount of toner on the recording sheet (image pattern), and the difference of the charged amount due to the irregular quality of a recording sheet and different humidity.
Accordingly, the recording sheet is not uniformly wound around the photoconductive drum in the axial direction of the photoconductive drum and thus the winding length is partially different.
Further, it is very difficult to make the peripheral speed of the photoconductive drum to be accurately equal to the feeding speed of the continuous sheet. As a result, in the situation when a continuous sheet is used as the recording sheet and the continuous sheet is fed by the fixing rollers, a difference of tension is caused in the width direction of the continuous sheet between the photoconductive drum and the fixing station. Thus the feeding amount of the continuous sheet in the width direction varies due to the difference of the tension applied thereto, and as a result, the skew of the continuous sheet occurs.
More specifically, the continuous sheet tends to proceed obliquely toward the side where the winding length is smaller because the portion of the continuous sheet in which winding length is long is more affected by the rotation of the photoconductive drum.
Once the skew arises, the position of the continuous sheet is taken in the fixing station (fixing roller pair) inclines more, and more and finally a jam is caused.